1. Field of the Invention
The present invention relates to a magneto-optical recording medium, in which the information can be written, reproduced, and erased by light, such as a laser or the like. More particularly, the present invention relates to an improvement in the environmental resistance of a magneto-optical recording medium, which comprises a transparent substrate made of synthesized organic resin as well as a recording layer formed thereon and having an easy axis of magnetization perpendicular to the film plane, and which is used for the magneto-optical recording and reproduction of information by the magneto-optical effect.
2. Description of the Related Art
Various studies are underway of optical recording media, since it features a high density- and high capacity-recording of information. Also, as the magneto-optical recording material is erasable, it is used for broad field of applications, and various related materials and systems have been disclosed. Actual utilization of the magneto-optical recording has been desired.
Various magneto-optical materials have been proposed, such as FeTb in Japanese Unexamined Patent Publication No. 52-31,703, FeTbGd in Japanese Unexamined Patent Publication No. 56-126,907, FeTbCo and FeCoDy in Japanese Unexamined Patent Publication No. 58-73,746, and FeNd in Japanese Unexamined Patent Publication No. 61-165,846. To be able to realize an actual utilization of these materials for erasable magnetooptical recording, it is desirable that the recording and reproducing characteristics thereof be further improved, and further, the durability thereof, including the oxidation-resistance be improved, in view of the probability of corrosion, such as oxidation and the like, of the majority of the above constituent materials of the recording layer.
Japanese Unexamined Patent Publication No. 59-110,052 proposes, in line with the above necessity for improvements, to prevent the recording layer from oxidation by sandwiching the recording layer of an optical recording element between two, oxygen-free films, at least one of which is a dielectric layer. Note, it is widely known to enhance the Kerr rotation angle by interposing a dielectric layer between the substrate and the recording layer. The dielectric layer must be free of oxygen and preferably is a nitride and fluoride, such as AlN, MgF.sub.2, ZnS, CeF.sub.3, AlF.sub.3.3NaF, Si.sub.3 N.sub.4, and the like. Among these, the nitride layer is considered to have an excellent resistance against environmental effects.
The present inventors investigated the nitride films of AlN, Si.sub.3 N.sub.4, and the like, which are considered to exhibit excellent environmental resistance among the dielectric materials, and discovered the existence of a slow film formation speed and a great strain in the film, which gave rise, particularly when depositing a multi-layer film on a plastic substrate, to a problem of peeling along grooves formed for the age resistance test. The peeling occurs because of an insufficient durability, which should be distinguished from the oxidation resistance.